The present invention relates to improvements in the method of attaching self-attaching or self-riveting female fasteners and installation tooling which results in an improved installation of the fastener in a metal panel and reduces the likelihood of installation tooling breakdown or breakage of the piercing tool and female die member.
Self-piercing and self-clinching or riveting female fasteners are used by the automotive industry and other mass production industries, such as the appliance industry, to attach various parts to a metal panel, such as a bracket, frame member, or the like. Examples of pierce nuts are shown in U.S. Pat. Nos. 3,299,500 and 3,315,138, assigned to the assignee of this application. The prior art also includes a number of self-riveting nuts, including nuts having an annular skirt or barrel portion, wherein the free end of the barrel portion is deformed radially outwardly in a die member to form a mechanical interlock between the nut fastener and the panel as disclosed, for example, in U.S. Pat. Nos. 4,398,239 and 4,018,257. The self-riveting nuts disclosed in such patents are, however, secured to a panel in a pre-pierced panel opening, requiring two separate operations. The method of installing the nut fastener disclosed in these patents also require very precise centering of the nut relative to the pre-pierced panel opening, wherein a spring-biased pin is received through the panel opening and the nut is centered on the pin, prior to installation. Further, the nut and panel installation does not have sufficient pull-out strength for many applications. U.S. Pat. No. 3,926,236, also assigned to the assignee of this application, discloses a method of attaching a nut wherein the panel is pierced by a punch which extends through the nut bore to pierce and secure the nut in a continuous operation. However, the fastener is not a riveting-type fastener having a barrel portion extending through the pierced panel opening. Self-riveting fasteners have also been used by the container industry, for example, for attaching a flange or tag ring for reception of a threaded plug as disclosed, for example, in U.S. Pat. No. 3,800,401.
This application and the above-identified related patents and patent applications disclose methods and apparatus for permanently attaching male and female fasteners to a panel, such as nuts and bolts, in a continuous operation. The preferred methods of installation and installation apparatus do not require pre-piercing of the panel, although self-attaching fastener elements of this type may be installed in a pre-pierced panel opening. Certain difficulties have, however, been experienced in installing female fastener elements in a panel, particularly where the barrel portion of the female element is used to pierce the panel opening. As will be understood, if the female fastening element is a nut-type fastener, the bore of the nut must remain clear of obstructions following installation for receipt of a stud or bolt. The method of installation and installation apparatus is also preferably suitable for mass production applications, such as used by the automotive and appliance industries.
Self-riveting fasteners of the type disclosed herein are particularly strong in resisting pull-out and cam-out of the fastener from the panel. Pull resistance is tested by threading a bolt in the threaded bore of the female fastener and pulling in a direction opposite to the direction of installation. The pull-out strength in many applications is greater than the tensile strength of the bolt. Cam-out is the strength of the installation to a cantilevered load. However, certain applications, such as automotive seat track and belt applications, also require improvements in push-through strength and resistance to torque. Resistance to torque is generally provided by barbs or ribs on the body of the fastening element which may result in stress cracks in the panel.
Further, in most applications, it is desirable for the fastener to be installed flush or slightly below the metal panel. These objects were achieved with the nut fastener and method of installation disclosed in the parent application of this application, Ser. No. 092,593; however, the piercing of the panel through the nut bore as disclosed resulted in further problems with the installation tooling, particularly in mass production applications. The female die which receives the punch and cooperates with the punch to pierce the panel and form the nut barrel into a mechanical interlock with the panel was subject to failure from stress cracks in the die post surrounding the bore. This problem could be alleviated by using a punch or quill having a smaller diameter; however, the punch was then subject to breaking. Further, a slight misalignment between the die button and the punch may result in a poor installation. Finally, stresses were generated in the panel surrounding the hole which receives the nut, sometimes resulting in stress cracking.
There was, therefore, a need to improve the method of installation and installation apparatus to eliminate these problems, including punch or quill and die button breakage, panel stress cracking, and misalignment between the punch and die button resulting from normal tolerance build-up in mass production applications. The method of assembly and installation apparatus of this invention accomplishes these additional objectives and results in an improved female fastener and panel assembly. Other advantages and meritorious features of this invention will be more fully understood from the following description of the preferred embodiments, appended claims and drawings, a brief description of which follows.